The main differences between these two bikes are
Group set- Ultegra is said to be better than 105 Agree Ultegra is marginally lighter, does that make it better? 105 is simply the old Ultegra anyway, and both work the same IME.
Wheel set- I do not know which one has better wheel set. Cube I think Probably nothing in it at this price point, both are a budget wheelset.
I have a Cube and my reason to buy the Cube was gear ratio. Cube's rear cassette is 12-30. Planet is 11-28. You may not see it as a big difference but for me it was an important consideration when going up the hills. I prefer 12-30 Cassettes are cheap as chips and simple to change, so don't let a ratio make your mind up for you. Or, speak to the retailer, I'm sure they'll swap it for your preference prior to collection.
I think geometry is also different. Cube is more sportive. Planet x is more race oriented, but I am not so sure about that. Me neither PX looks 'racier' due to daft saddle angle and post height IMO, but if you look at the at Cube stem you can see that will also be an arse up riding position too.
